There is a Contact sharing rule but it is not giving Users access to Contact records as expected.
Check the Organization-Wide Sharing Defaults for Contacts from the Sharing Settings.
In Salesforce Classic
Click on Setup | Security Controls | Sharing Settings
In Lightning Experience
Click on the Gear icon | Setup | Security | Sharing Settings
If Contact sharing is set to 'Controlled by Parent,' then sharing access to Contacts is controlled by access to the related Account record.
When Contact sharing is set to 'Controlled by Parent,' Contacts may not be shared independently from Accounts, so Contact sharing rules are not applied (even if they are defined).
To address this, you can:
Related Content:
'View All' permission on parent object does not automatically give access to child object
Define Sharing Rules Trailhead Module
Create Sharing Rules Trailhead Module
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